What is/are the difference(s) between the two following variables? (Except their names)-CodePudding

char *s1 = "";

char *s2 = NULL;

I just want to know the difference

CodePudding user response：

One is pointing to the first element of an array of a single char element, the element being the string null-terminator character '\0'.

The other variable is initialized point to NULL which means it doesn't point anywhere, really.

Slightly simplified, the definition

char *s1 = "";

is kind of equivalent to

char private_array[1] = { '\0' };
char *s1 = &private_array[0];

It might seem confusing to have both a string null terminator, and a generic null pointer, but it will clear itself out with more experience.

Also note that in C all literal strings (even "") are not modifiable, they are in essence read only. That's why it's recommended to always use const char * to point to literal strings.

CodePudding user response：

There's a very big differnce using "" leaves '\0', whereas NULL leaves the pointer as ((void *)0).

This can be tested via dereferencing the pointer like:

NOTE: Dereferencing the pointer means accessing its inner values or elements, which is done using * unary operator or [] operator in C/C .

""

#include <stdio.h>

int main(void){
    char *s1 = "";
    printf("%d", *s1);
    return 0;
}

I'm printing the integer representation of *s1 because '\0' can't be seen on the terminal.

Ouput:

And program returned 0 which means success.

NULL

#include <stdio.h>

int main(void){
    char *s2 = NULL;
    printf("%d", *s2);
    return 0;
}

The above program printed nothing on stdout, but it returned 139, which means program crashed before it exited (segmentation fault).

You can try it online.

CodePudding user response：

An empty string has a single element, the null character, '\0'. That’s still a character, and the string has a length of zero, but it’s not the same as a NULL, which has no characters at all.